In recent years, electrophotographic image forming apparatuses such as laser printers, digital copiers, and laser facsimiles were provided with a post-processing device (finisher) stapling a sheet bundle. When sheets discharged from an image forming unit are subjected to the post-processing, the throughput of a stapler increases depending on the timing for stapling. Accordingly, even when sheets are discharged from the image forming unit with a usual sheet interval, the stapling of the stapler is delayed. Therefore, for example, in JP-A-4-148993, a finisher includes a mechanism buffering (holding) two or three sheets, and a delay time is provided between a sheet to be subjected to the post-processing and a sheet to be subjected later to the post-processing so as to smoothly staple the sheets at the time of forming an image at a high-speed.
For example, in JP-A-2006-27769, the control means for controlling to switch a first sheet gap control for forming an image with a sheet gap not including a post-processing time, a second sheet gap control for forming an image with a sheet gap including the post-processing time, a first post-processing carrying control for temporarily stopping a first sheet of a subsequent job in a carrying path before an intermediate stacker during performing the post-processing on a sheet bundle of a previous job, overlapping a first sheet and a second sheet of the subsequent job with each other, and carrying the overlapped sheets before the intermediate stacker after completing the post-processing of the previous job, and a second post-processing carrying control for temporarily stopping the first sheet of a subsequent job in the carrying path before the intermediate stacker during the post-processing on the sheet bundle of the previous job and carrying the temporarily-stopped sheet to the intermediate stacker after completing the post-processing of the previous job is provided.
However, in JP-A-4-148993, there is no problem when the processing is continued with the same sheet gap, but an extra standby time is added when a mode with a long sheet gap is switched to a mode with a short sheet gap, thereby deteriorating the performance of the image forming process. For example, when the two-position stapling is continued, there is no problem. However, when the two-position stapling is switched to the one-position stapling, the delay time for the two-position stapling is taken, thereby adding the extra standby time.